A cellular communication system can support bi-directional communication for multiple users by sharing the available system resources. Cellular systems are different from broadcast systems that can mainly or only support unidirectional transmission from broadcast stations to users. Cellular systems are widely deployed to provide various communication services and may be multiple-access systems such as Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, Orthogonal FDMA (OFDMA) systems, Single-Carrier FDMA (SC-FDMA) systems, etc.
A cellular system may support broadcast, multicast, and unicast services. A broadcast service is a service that may be received by all users, e.g., a news broadcast. A multicast service is a service that may be received by a group of users, e.g., a subscription video service. A unicast service is a service intended for a specific user, e.g., a voice call. Group communications can be implemented using unicast, broadcast, multicast, or a combination of each.
The capacity of the unicast air interface is a limiting factor for interactive group communication for a large group of users in a mobile environment and/or in a small geographical region, such as a push-to-talk group call between a large number of users within a stadium. As the group becomes larger it is generally more efficient to use multicast services. Using multicast channels such as Broadcast and Multicast Service (BCMCS) or evolved Multimedia Broadcast Multicast Services (eMBMS) solves the problem of the limited air interface, but requires a very expensive infrastructure and core network, and as such, may not be a practical solution for most operators. In addition to the air interface limitation, the scalability of the group communication server is also a limiting factor for very large group call communication.